The timer IC555 is used as a switching pulse oscillator. IC 555 configured as Astable Multivibrator to give continuous switching pulse. Two switching transistors TIP41A (NPN) and TIP42A (PNP) drives the transformer T1 according to the pulse input at the base. The transformer is 230V primary to 9V secondary but connected in reverse, So it can react as step up transformer. We can apply +5V to +15V DC bias to this circuit and get 110V to 230V AC with 50Hz to 60Hz frequency. But output may not pure sine wave as the PWM inverter output, it gives only pulsated AC.
The output frequency of this circuit can be varied by varying VR1 resistor.
Use this formula to calculate the output AC supply and frequency, here R2 = R2+VR1 from given circuit. Use heat sink for Transistors to avoid over heat.